Direct fuel-injection engine

Abstract

In a direct fuel-injection engine equipped with a pentroof-shaped piston, cross-sectional shapes containing a piston central axis (Lp) of a cavity recessed in a central part of the piston having a top face with the height varying in the circumferential direction are set so as to be basically identical at each position in the circumferential direction (see broken line). Since the sizes of the cross-sectional shapes of the cavity thereby become different in a region further on the piston central axis (Lp) side than intersection points (e 1 , d 1 ) in the vicinity of a fuel injection point (Oinj), in order to compensate for this, by correcting in a cross section that is away from the piston pin direction the cross-sectional shape of the broken line into the cross-sectional shape of a solid line so that it enlarges, the size of the cross-sectional shape at each position of the circumferential direction of the cavity is thereby made more uniform, thus making the conditions in which fuel and air are mixed uniform and improving the engine output and reducing harmful exhaust materials.

The invention claimed is: 
     
       1. A direct fuel-injection engine comprising a piston having a top face with a height in a piston central axis direction that varies in the circumferential direction, a cavity recessed in a central part of the top face of the piston, and a fuel injector for injecting fuel into the cavity,
 wherein the shape of an inner wall face of the cavity is set so that, with N as A natural number of 2 or more, when the cavity is divided into N virtual cavity sections by the inner wall face of the cavity and N half planes extending radially from the piston central axis and having equal included angles, the volumes of the virtual cavity sections are substantially equal. 
 
     
     
       2. The direct fuel-injection engine according to  claim 1 , wherein
 the fuel injector comprises a plurality of fuel injection axes spaced in the circumferential direction, 
 a cross-section of the cavity passing through an nth fuel injection axis is defined as a fuel injection cross-section, 
 an intersection point of the fuel injection cross-section with the opening Peripheral edge of the cavity is defined as a first specific point, 
 a second specific point is present on a line that passes through the first Specific point and is parallel to a lower face of a cylinder head in the fuel injection cross-section, 
 a third specific point is present on a bottom wall portion of the cavity in the fuel injection cross-section, 
 the second specific point is at a position closer to the piston central axisthan the first specific point, 
 the third specific point is at a position closer to the piston central axisthan a position of the maximum outer diameter of the bottom wall portion of the cavity, 
 a cross-sectional shape surrounded by a path connecting the first and Second specific points by a line along the lower face of the cylinder head in the fuel injection cross-section, a path connecting the first and third specific points along a wall face of the cavity in the fuel injection cross-section and a path connecting the second and third specific points by the shortest straight line, the cross-sectional shape being substantially equal for each fuel injection cross-section, is defined as a reference cross-sectional shape, and 
 the volume of each virtual cavity section is made substantially equal by varying the shape of the inner wall face of the cavity so that the reference cross-sectional shape enlarges for the fuel injection cross-section passing through a fuel injection axis that is present in a direction where the height, in the piston central axis direction, of the top face of the piston is low. 
 
     
     
       3. The direct fuel-injection engine according to  claim 2  wherein, of the path connecting the first and third specific points, the shape of a section from the lowest part of the path connecting the first and third specific points to the third specific point comprises a diagonal section. 
     
     
       4. The direct fuel-injection engine according to  claim 2 , wherein the number N of the virtual cavity sections is equal to the number of the fuel injection axes. 
     
     
       5. The direct fuel-injection engine according to  claim 4 , wherein the bisector of the included angle coincides with the fuel injection axis when viewed in the piston central axis direction. 
     
     
       6. The direct fuel-injection engine according to  claim 1 , wherein the volume of the virtual cavity sections includes the volume of a portion sandwiched by the top face of the piston at top dead center and a lower face of a cylinder head. 
     
     
       7. The direct fuel-injection engine according to one of  claims 1  to  5 , wherein the shape of the top face of the piston is a pentroof shape having two inclined faces that are inclined with a ridgeline parallel to the axis of a piston pin interposed there between.